This invention concerns digital radio communication. It applies in particular to professional mobile radio communication systems.
In general, two requirements are to be satisfied in this field.
On the one hand, a radio communication network must provide radio coverage which is as extensive as possible, while using the smallest possible number of base stations.
On the other hand, the radio communication system must permit the maximum possible number of simultaneous communications in a given frequency band. This calls for a spectral efficiency which is even higher when the environment has a high communication density, as is often the case in urban areas.
The two requirements referred to above are difficult to reconcile, since they correspond to conflicting conditions for the definition of the parameters of the radio communication system. Indeed, a reduction in the spectral occupancy of the communications entails a reduction in the sensitivity of the radio receivers. This causes a reduction in the range of transmitters, and the need to increase the number of base stations for a given coverage and quality.
An object of the present invention is to overcome these drawbacks, by proposing a particular method of organising the radio transmission channels.
The invention thus proposes a digital radio communication station including a coded speech signal source having a given digital rate, and modulating means for producing a signal from the coded speech signal provided by the source. According to the invention, the station further includes a redundancy encoder having a coding rate of the form K/N, where K and N are integers such that 0 less than K less than N, and control means for selecting an operating mode of the station from at least a first operating mode where the modulating means directly receive the coded speech signal provided by the source and convert it into a radio signal occupying a determined fraction of time on a carrier frequency, and a second operating mode where the redundancy encoder receives the coded speech signal provided by the source and the modulating means receive the output signal from the redundancy encoder and convert it into a radio signal occupying N/K times said determined fraction of time on a carrier frequency.
The second operating mode provides greater sensitivity of the stations in communication, at the expense of greater occupancy of the available spectrum resources. Thus, the same station (base station or portable terminal) using the same coded speech signal source and the same modulating means may be used advantageously in the first mode if the surrounding traffic is very dense (typically in urban areas), and in the second mode in areas where the traffic is less dense and maximum radio coverage is looked for.
The invention is advantageously applied to frequency-division or code-division multiple-access (FDMA or CDMA) radio communication systems. In this case, the above-mentioned determined fraction of time on a carrier frequency is equal to K/N. The time occupancy of the carrier is divided into uniform time intervals. A communication uses 100% of these time intervals in the second mode, and only a proportion K/N in the first mode. It is convenient that these time intervals each correspond to a frame of the coded speech signal.
The invention may also be applied to time-division multiple-access (TDMA) radio communication systems. A communication may then be allocated a larger or smaller fraction of time on a given carrier by reserving a suitable number of elementary TDMA timeslots on the carrier.